Short term fuel trims on both banks read around 5% but the Long term fuel trims read 19 and the other 21% with no check engine light.

I can’t seem to figure out if it’s a fuel delivery problem or a vacuum leak. I used a homemade smoke machine and found a couple of leaks and fixed them but still have this lean issue. It’s causing me to fail emissions due to unburnt fuel causing high HC.

The only other clue is, when the car is at idle, exactly at 24 seconds, it goes into open loop despite having the car warmed up. Apply throttle, then goes into closed loop. Then, at idle at 24 seconds, it goes into open loop where it stays until throttle is applied again.

Checked the ECT, and the IAT sensors and all tested fine per the FSM.

 

One other thing I noticed, is that on occasion, when I start the car it will be in open loop, and after 30 seconds or so, it will go into closed loop for a couple of seconds then, switch back into open loop, despite the car being warmed up for several minutes thereafter.

 

Assuming all other regular maintenance is taken care of........

1st, I suggest putting your car on a TCW3 diet. When you first use it, alot of carbon will blow out through the exhaust. The biggest benefit IMO of TCW3 is lubricating the injectors to get them working optimally.....injecting more accurately according to what the ECU expects them to do.

https://maxima.org/forums/fluids-lub...itive-new.html

2nd, after a couple of days/weeks of TCW3, depending on how you drive, I would then replace your primary O2 sensors, especially if you've never replaced them before. They get tired and give inaccurate readings to the ECU. Dont want to get the new sensors all gunked up with carbon.

After that your car should be putting out the cleanest exhaust and best power it ever has.

 

I have been using TCW3, for a month now, and the O2s on both banks work fine oscillating from ~.9-.1 volts.

Initially, after I had used a Y-type vacuum connector before the EGRC solenoid to the EGR, applying supplemental vacuum from the EGR-BPT valve to the EGR, I had high HC. I thought this was due to the O2’s thinking there was unmetered air, and compensated by adding more fuel to achieve the optimal AFR. Since then, I have reconnected the vacuum hoses back to stock, and read the fuel trim levels on the GST (Generic Scan Tool). They were the same. STFT were normal, but the LTFT were +15-+19 on both banks.

Then, when I scanned it again, it was reading the opposite. The LTFT were reading around +9 on both banks and the STFT was ~15-19%.It didn’t matter if I had the additional vacuum line or not, the readings from the STFT were way off, despite numerous drive cycles.

Then curiously, it switched back to STFT levels being normal and LTFT ~+15+19. My mechanic chalked it up to being leaky fuel injectors. The one clue he had was that when he was performing the emissions test, the HC numbers were fine when he kept in 1st gear during the 15 mph test, as opposed to switching it to 2nd gear. Could this happen to be related to my open loop at idle problem? Because when you go full throttle, it switches back from closed loop to open loop?

 

Since you tested all the circuits and they come up within spec, then maybe you have grounding issues so the ecu is not getting clean readings. You should clean all your contacts in the engine bay.

As for your primary O2s , just because you can see them oscillating doesn't mean they're good. It's like looking at a tire to judge if they have the right tire pressure. If you've never changed your primary O2s, or they have more than 100k miles on then, change them.